1

OUTER SHIELD/GROUND

SIGNAL CONDUCTOR

GUARD

+

-

+1

IN

Guard

Guard

Driver

VREF

+

-

OUT

LMP7721

www.ti.com

SNOSAW6E – JANUARY 2008 – REVISED DECEMBER 2014

Application Information (continued)

Figure 48. Guarding the Inverting or Transimpedance Configuration

The gain of the buffer should be slightly less than one to prevent oscillations and should be current limited to

protect against short circuits. The buffer amplifier should also be capable of driving large capacitive loads. To

satisfy these two requirements, a small series output resistor is usually placed on the buffer output in the range

of 100

Ω to 1 kΩ.

For optimum results, the guard should completely enclose the input circuitry within a conductive "cocoon",

including above and below the circuitry. A cover or shield connected to the guard should protect the circuitry

above (or below) the PC board. Do not forget about thru-hole devices (like leaded photodiodes or connectors)

that may expose high-impedance nodes to the opposite side of the board.

The guard trace should not be relied upon as the only method of shielding. A ground plane or shield should

surround and protect the guard from large external leakages and noise, as the guard trace has the potential to

couple noise back into the input. For more information on guarding, please see the articles referenced in Related

Documentation.

8.1.2 Use Triaxial Cable

A triaxial cable or connector is similar to a coaxial cable or connector and is often referred to as “triax”. The

triaxial cable extends the guard protection through the length of the cable by adding a second internal guard

"shield" around the center conductor in addition to the outer ground shield. Figure 49 shows the structure of the

triax connector.

Figure 49. The Structure of a Triax

8.1.3 Properly Clean the Assembly

Proper cleaning of the board is very critical to providing the expected sub-picoamp performance. Properly

cleaning the board and components takes a few extra steps over conventional board cleaning methods. Leftover

flux residue, moisture and cleaning solvent residues will severely degrade the low-current performance.

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